1. Field of the Invention
The invention relates to a reflector used for a light source material of a liquid crystal video projector.
2. Description of Related Art
A reflector, which is used for a light source material of a liquid crystal video projector, reflects with high efficiency radiation light e.g. produced by a short arc-type metal halide lamp, converts it into parallel light beams and conducts it to a liquid crystal panel.
In the case of a colour projection by means of a reflector reflected, parallel light beams are dispersed through a dichroic mirror in red, blue and green and in each case introduced into a liquid crystal panel. An image formed on the liquid crystal panel is supplied to a projection lens, whose passage light is projected onto a screen.
Such a reflector is made from a light-transmitting, heat-resistant glass and has a substantially parabolic internal shape, in order to obtain parallel light beams. There is a point source of light in a focus position of the reflector.
By depositing a dielectric multifilm coating on one surface of the paraboloid only visible light beams are reflected and projected in the direction of the liquid crystal panel, whereas neither infrared beams nor ultraviolet beams are projected in the direction of said liquid crystal panel, because they are not reflected by the reflector and instead pass through the same.
FIG. 1 shows an example of a graph of a spectral transmittance, in which radiation light from a light source has vertically entered a reflector on which is deposited a multifilm coating of titanium dioxide (TiO.sub.2) and silicon dioxide (SiO.sub.2). In the graph the abscissa axis represents a radiation wavelength from the light source and an ordinate axis the transmittance through the reflector.
This makes it clear that light with a wavelength of approximately 450 to 700 nm is reflected to a considerable extent and has a transmittance of almost 0% and that light outside this wavelength range is not reflected and passes through. If a wavelength, which on one short wave side has a spectral transmittance of 20% is designated .lambda..sub.1, and a wavelength, which on one wavelength side has a spectral transmittance of 50% is designated .lambda..sub.2, then .lambda..sub.1 is at approximately 420 nm and .lambda..sub.2 at approximately 760 nm. A reflection wavelength width (.lambda..sub.2 -.lambda..sub.1) is at approximately 340 nm.
The reason why on the short wave side a wavelength with a spectral transmittance of 20% is taken as the standard and on the long wave side a wavelength with a spectral transmittance of 50% is taken as the standard, is that account is taken of the fact that the incident light is partly absorbed by the reflector and that this absorption is more pronounced in the case of light with a short wavelength than light with a long wavelength.
In order to obtain a good projection image in the case of such a liquid crystal video projector, it is necessary that one face of a screen is irradiated in a uniform manner and that there is no colour shading.
However, in practice, in the peripheries of the screen diagonal dark blue portions appear. In FIG. 5 reference numbers 100 indicate measurement points. This colour shading is illustrated by means of dots in FIG. 5. The possible reason for this is the angle of incidence of the radiation light from the light source in the reflector. This means that light beams, which are reflected with a small incidence angle in the centre of the reflector irradiate the centre of the screen, whereas light beams with a large incidence angle reflected in the reflector periphery irradiate the periphery of the screen. These incidence angle differences give rise to reflection characteristic differences between a p-polarized light and an s-polarized light of the incident light, which probably influence the colour shading.
FIG. 1 is a graph of a spectral transmittance in the case of normal incidence in the reflector (i.e. with an angle of incidence of 0.degree.), whereas graphs of the spectral transmittance at an incidence angle of 25.degree. and an incidence angle of 50.degree. in the same reflector are shown in FIGS. 2 and 3 respectively.
This makes it clear that a p-polarized light and a s-polarized light in the case of an incidence angle of 0.degree. give rise to no reflection characteristic difference. However, a reflection characteristic difference between a p-polarized light and a s-polarized light increases the larger the incidence angle and that in addition a reflection wavelength width moves towards a short wave side.
A liquid crystal display panel is structured in such a way that a liquid crystal cell including a number of picture elements is fixedly sandwiched between a p-polarizing plate and an s-polarizing plate. Light beams which pass through the centre of this liquid crystal panel are, as described hereinbefore and as a result of a small reflection characteristic difference between a p-polarized light and a s-polarized light, projected onto a screen in a state in which there is virtually no colour shading.
However, due to a large reflection characteristic difference between a p-polarized light and a s-polarized light, in the periphery of the liquid crystal panel the light beams passing through the same have a colour shading in the manner described hereinbefore. As these light beams are projected onto the screen, as a result of this said colour shading is also projected onto the screen.